Means and method for controlling a solvent refining unit to provide optimum yields of refined oil and extract oil

ABSTRACT

A solvent refining unit is initially operated at a predetermined solvent dosage and a predetermined extract-mix temperature to yield refined oil of a desired quality from charge oil. A quality constant is determined from the initial operation of the refining unit. Limitations on a selectivity characteristic of the solvent, defining an area of feasible operation, are determined from various maximum operating characteristics and equations, hereinafter disclosed. The charge oil flow rate is repeatedly increased in a stepping fashion and an actual solvent selectivity characteristic for each step is calculated using the solvent dosage and the quality constant. An earnings position for the refining unit is determined for each step. When the earnings position for a current step is less than the earnings position for the previous step, or when the actual solvent selectivity characteristic for the current step does not lie within the area of feasible operation, the charge oil flow rate is decreased to the flow rate for the next previous step.

United States Patent 1 Woodle 11 3,718,809 1 Feb. 27, 1973 1 MEANS ANDMETHOD FOR CONTROLLING A SOLVENT REFINING UNIT TO PROVIDE OPTIMUM YIELDSOF REFINED OIL AND EXTRACT OIL [75] Inventor: Robert A. Woodle, PortArthur,

Tex.

[73] Assignee: Texaco Inc., New York, NY. [22] Filed: April 21, 1971[21] Appl. No.: 136,003

[52] U.S. Cl. ..235/151.12, 208/33, 208/311 [51] Int. Cl ..G06g 7/58,C10g 21/00 [58] Field of Search ..235/151.l2, 151.35; 208/27-28, 33, 36,311; 23/230 R, 203 A, 255 E [56] References Cited UNITED STATES PATENTS3,458,431 7/1969 Nixon ..208/33 3,546,107 12/1970 Brown et al...235/151.12 X 3,549,514 12/1970 Brown et a1. ..235/l.l2 X 3,554,8961/1971 Bozeman, Jr. et al. ..208/33 X 3,565,786 2/1971 Brown et a1...235/15l.l2 X 3,458,432 7/1969 Woodle ..208/36 REFINING 5 TOWER 4 came:

"- FRC SWITCH 6 5 COMPUTER COMPARATOR SWITCH SOURCE OF DC VOLTAGESPrimary Examiner-Eugene G. Botz Assistant Examiner-Jerry SmithAttorney-Thomas H. Whaley and Carl G. Ries 57 ABSTRACT A solventrefining unit is initially operated at a predetermined solvent dosageand a predetermined extract-mix temperature to yield refined oil of adesired quality from charge oil. A quality constant is determined fromthe initial operation of the refining unit. Limitations on a selectivitycharacteristic of the solvent, defining an area of feasible operation,are determined from various maximum operating characteristics andequations, hereinafter disclosed. The charge oil flow rate is repeatedlyincreased in a stepping fashion and an actual solvent selectivitycharacteristic for each step is calculated using the solvent dosage andthe quality constant. An earnings position for the refining unit isdetermined for each step. When the earnings position for a current stepis less than the earnings position for the previous step, or when theactual solvent selectivity characteristic for the current step does notlie within the area of feasible operation, the charge oil flow rate isdecreased to the flow rate for the next previous step.

12 Claims, 11 Drawing Figures EXTRACT on.

S COMPUTER an r COMPUTER COMPUTER EARNINGS COMPUTER V10 s s SOURCE OF DCVOLTAGES 35 MONOSYABLE SAMPLE MULTI AND 111 BR ATOR H'J L [I COMPARATORPATEM'EDFEBZYIQH 7 718,809

SHEET 10F 6 FIG. IA' A REFINING TOWER WATER CHARGE OIL TRC B COMPUTERCOMPARATOR SWITCH 6O SOURCE OF DC VOLTAGES 'PATENTED 3.718.809

SHEET 2 OF 6 E A FIG. IB

n STRIPPER L LEI I I6 DEWAXING =REFINED MEANS OIL I WAX E STR'PPEREXTRACT on. 5A b7 v 4 E, as) I I L CR0 CT S 2 COMPUTER E COMPUTERCOMPUTER l0 Lv, A A v 4 S i L 32 )L EIO E EI2 l7 4 2 no I I I CAEARNINGS F v COMPUTER COMPUTER j p EI4 4 TEM 62 I COMPUTER ,1/ v

3 20 2l 22 2| I 7 7 L Io 6 vs s 1 SOURCE OF DC VOLTAGES TV I35 I22 LswwcTl?- 36 I44 FLIP MONOSTABLE SAMPLE MULTI AND FLOP VIBRATOR HOLD (I36I27 i COMPARATOR- PATENTEUFEBZYIQIS 3 71 09 SHEET 30F 6 FIG. 2

MAXIMUM EXTRACT OIL FLOW RATE 8 MAXIMUM EXTRACT MIX TEMPERATURE FEASIBLE0.05 REGION V 0.03 MAXIMUM REFINED- WAXY OIL FLOW RATE 1 l v l s I I I60 I00 200 500 I000 2000 PATENTEB FEB2 7 I975 SHEET 0F 6 FIG. 4

vll

EIO

R F l D O B FIG.

I 0 COMPUTER 1 FIG. 6

C 0 COMPUTER PATENTEDFEBZYW 3,718,809

SHEET 5 BF 6 X l 81 E l0 9| l SQUARE cfisaJT I F 7 V L as EARNINGSCOMPUTER-1 f u I {IMHO PATENTED FEB27I975 SHEET 8 BF 6 FIG. 10

' /REFERENCE TEMPERATURE C SOLVENT SELECTIVITY CHARACTERISTlC MEANS ANDMETHOD FOR CONTROLLING A SOLVENT REFINING UNIT TO PROVIDE OPTIMUM YIELDSOF REFINED OIL AND EXTRACT OIL BACKGROUND OF THE INVENTION 1. Field ofthe Invention The present invention relates to control systems and, moreparticularly, to an automatic control system for use in an oil refinery.

2. Description of the Prior Art Heretofore, optimum yield controlsystems for solvent refining units such as the one disclosed in U.S.application Ser. No. 96,193, filed Dec. 8, 1970, now U.S. Pat. No.3,666,931 by R.A. Woodle, inventor of the present invention, andassigned to Texaco Inc., assignee of the present invention, useequations to determine the charge oil flow rate and refiningtemperature, i.e. the extract-mix temperature, that should be used foroptimum yield and to control the refining unit accordingly.

The present invention provides a more simplified control system. Thesystem of the present invention differs in concept from theaforementioned U.S. application by using an area of feasible operationas defined by limitations on a refining parameter and controlling therefining unit to provide optimum yields while operating within the areaof feasible operation.

SUMMARY OF THE INVENTION A system controls a solvent refining unit whichtreats charge oil with a solvent in a refining tower to yield raffinateand extract-mix. Strippers in the refining unit separate the solventfrom the rafiinate and from the extract-mix to provide refined waxy oiland extract oil, respectively. The solvent is returned to the tower andthe refined waxy oil is subsequently dewaxed to provide refined dewaxedoil. The control system includes a circuit which senses refiningconditions and provides corresponding signals. A plurality of networksare connected to the sensing circuit. One network provides signalscorresponding to limitations of a refining parameter in accordance withat least one sensed condition signal. Another network provides a signalcorresponding to the actual value of the refining parameter inaccordance with some of the sensed condition signals. Another networkprovides a signal corresponding to the relationship of a presentearnings position of the refining unit with a previous earnings positionin accordance with some of the sensed condition signals. A signalcorresponding to a quality constant for a desired quality refined oil isapplied to control means along with the signals from the networks. Thecontrol means control the refining unit in accordance with appliedsignals to provide optimum yields of the desired quality refined oil andthe extract oil from the charge oil.

One object of the present invention is to control a solvent refiningunit to provide optimum yields of refined oil and extract oil fromcharge oil.

Another object of the present invention is to use the relationship of asolvent selectivity characteristic and the solvent dosage along with theearnings position of the solvent refining unit to control the solventrefining unit to provide optimum yields of refined dewaxed oil andextract oil.

Another object of the present invention is to determine a qualityconstant for charge oil being refined in a solvent refining unit so thatthe quality constant may be used in controlling the refining operation.

The foregoing and other objects and advantages of the invention willappear more fully hereinafter from a consideration of the detaileddescription which follows, taken together with the accompanying drawingswherein one embodimentof the invention is illustrated by way of example.It is to be expressly understood, however, that the drawings are forillustration purposes only and are not to be construed as defining thelimits of the invention.

DESCRIPTION OF THE DRAWINGS FIGS. 1A and 13, when matched along lineA-A, provide a simplified block diagram of a system, constructed inaccordance with the present invention, for controlling a solventrefining unit, which is also partially shown in schematic form.

FIG. 2 is a logrithmic graph of solvent selectivity characteristicversus solvent dosage, for different limiting refining condition, whichdefine a feasible region of operation for the solvent refining unitpartially shown in FIG. 1.

FIGS. 3 through 9 are detailed block diagrams of the S computer, the Bcomputer, the C, computer, the C no computer, the C computer, the Tcomputer and the earnings computer, respectively, shown in FIGS. 1A v&18.

FIG. 10 is a graph of extract-mix temperature versus solvent selectivitycharacteristic for furfural solvent.

DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown asystem for controlling a conventional type solvent refining unit foroptimum operation. The rate of flow of charge oil entering a refiningtower 3 is controlled so as to regulate the flow rates of refined waxyoil and extract oil. The temperature of extract-mix, as the extract mixleaves the refining tower 3, is also controlled to affect the yield ofrefined dewaxed oil and the extract oil. The rate of the charge oilentering the refining tower 3 in a line 4 is sensed and controlled byconventional types sensing element 5, flow recorder 6 and valve 2.Sensing element 5 provides a signal to controller 6 corresponding to theflow rate of the charge oil in line 4. Controller 6 operates valve 2 tocontrol the rate of flow of the charge oil to tower 3 in accordance witha difference between the signal from sensing element 5 and the positionof its set point.

Although not shown, for ease of explanation, the charge oil and refiningsolvent entering tower 3 through lines 4 and 7, respectively, have beenheated to a predetermined temperature. Tower 3 contains packing 8 wherethe charge oil and solvent are contacted in counter flow effecting theextraction of low viscosity index constituents of the charge oil.Raffinate including refined waxy oil and a small amount of dissolvedsolvent is withdrawn through a line 10.

A temperature gradient is maintained in tower 3 by means of a coolingcoil 11 having cooling water flowing through it. The temperature intower 3 is sensed by conventional type sensing means 12 which provides acorresponding signal to a temperature controller 14.

Temperature recorder controller 14, which may be of a type well known inthe art, operates a valve 15 in accordance with a difference between thesignal from temperature sensing means 12 and its set point position.Valve 15 controls the rate of flow of the cooling water so as to controlthe temperature in tower 3.

Rafiinate in line enters a stripper which strips the solvent from theraffinate to yield the refined waxy oil. The solvent is returned totower 3 by line 7, while the refined waxy oil is provided to dewaxingmeans 16 through a line 17 Dewaxing means 16 removes the wax andprovides refined dewaxed oil for storage and blending with productlubricating oil.

Sensing means 5A and a conventional type flow transmitter 20 measuresthe rate of flow of the refined waxy oil from stripper 15 and provides acorresponding signal E Elements having a numerical designation with asuffix are identical in operation as elements having the same numericaldesignation without a suffix.

Extract-mix comprising solvent and dissolved low viscosity indexconstituents of the charge oil is withdrawn from tower 3 through a line22 at a temperature T controlled by cooling coil 11. The extract-mix inline 22 is passed to a stripper 23 where the solvent is stripped fromthe extract oil which is discharged through a line 25. The recoveredsolvent is withdrawn through line 7 for return to tower 3 and reuse. Theflow rate of the solvent in line 7 is maintained at a maximum and thesolvent dosage is controlled by controlling the fiow rate of the chargeoil in line 4. Sensing means 5B and a flow transmitter 20A senses therate of flow of the extract oil in line 25 and provides a correspondingsignal E Initially the refining of the charge oil is done at a solventdosage and temperature combination selected from various combinations ofsolvent dosages and temperatures that yield the desired quality ofrefined oil. A three pole, two position switch when in the positionshown in FIG. 1 permits signals E E E, from an external signal source,which is not shown, to pass. Signal E adjusts the set point oftemperature recorder controller 14 to correspond to a selectedtemperature during the initial operation. Signal E has pulses whichchange the set point of flow recorder controller 6 in a directiondetermined by signal E so that the flow rate of charge oil in line 4 isof the proper value for the solvent dosage required during the initialoperation.

Yield alone does not define the quality of the refined oil. The qualitycan vary for a constant yield, depending upon the selectivity of therefining conditions. Selectivity is the relative ability of the refiningsolvent to discriminate between the undesirable or extractablemolecules, and the desirable raffinate-quality ones. The less oil thereis in the solution in the extract-mix, the more selective the separationsince the solvent is more nearly pure solvent. Selectivity improves asthe extraction temperature is reduced, thereby reducing the relativesolubility of the oil in the solvent. However, since each volume ofsolvent contains less oil at the lower temperature, more volumes ofsolvent must be used to achieve the desired degree of refining. As theconditions are made more selective, by reducing the extractiontemperature and increasing the solvent/oil ratio, the yield of refinedoil having the specific quality improves, but the production rate islowered. Thus there is an economic optimum selectivity.

Economic optimization involves the interrelationship between yield andproduction. Any given solvent refining unit has three main productionlimitations: solvent flow rate capacity, refined oil flow rate capacityand extract oil flow rate capacity. These limitations define an area offeasible operation for a solvent selectivity C =EO /(SOL+EO (l) whereB0,, is the maximum possible flow rate of extract oil in line 25 asdetermined from the physical design of the solvent refining unit and SOLis the flow rate of the solvent in line 7. In developing FIG. 2, amaximum extract oil flow rate of 2,600 barrels per day (BPD) and amaximum solvent flow rate of 17,000 BPD are used. Since the solvent flowrate SOL is at a maximum, a voltage corresponding to that flow rate maybe provided instead of sensing the actual flow rate in line 7.

The line corresponding to the limitation on the solvent selectivitycharacteristic C no due to the maximum possible refined waxy oil flowrate RO which for FIG. 2 is 6,000 BPD, is determined from the followingequation:

(SOL) S(R0M) S(SOL) +100(SOL) -s(R0M CT: R Mrso) where T is a referencetemperature, which is explained in detail hereinafter, for the solvent,T is the miscibility temperature, and for FIG. 2 is 230F., of the chargeoil and A is a constant characteristic of the charge oil as determinedby laboratory analysis which for FIG. 2 has a value of 186. The methodof determining A is disclosed in US. Pat. No. 3,458,432. The exponent nis a characteristic constant related to the type of solvent and rangesin value from 0.42 to 0.67. An article entitled Figure Solvent Extractof Heavy Oils" written by the inventor and published in HydrocarbonProcessing, July 1966, Vol. 45, No. 7, page 133 relates different valuesof n for corresponding solvents. The miscibility temperature is thattemperature at which the charge oil dissolves completely in the solvent.The miscibility temperature may be determined in the laboratory for aparticular charge oil.

Referring to FIGS. 1 and 3, an S computer 32 provides a signal Ecorresponding to the actual solvent dosage S in accordance the refinedwaxy oil and the extract oil flow rate signals E and E respectively, andthe following equation which by definition is the solvent dosage, i.e.the ratio of the solvent percent volume to the charge oil percent volumemultiplied by 100:

SOL

where CO, R0 and E0 are the flow rates of the charge oil, the refinedwaxy oil and the extract oil, respectively. Computer 32 includes summingmeans 34 summing signals E E from flow transmitters 20 and 20A,respectively. A direct current voltage V, from a source 36 of directcurrent voltages is divided by the sum signal from summing means 34 by adivider 38. A multiplier 41 multiplies the signal from divider 38 with adirect current voltage V, from source 36 to provide signal E The actualsolvent dosage signal E is applied to a computer 44 which uses signal Ealong with temperature signal E, from temperature recorder controller 14to provide a signal E corresponding to the quality constant B. Referringto FIGS. 1 and 4, the B constant signal B is provided after it has beendetermined by laboratory analysis that the refined oil is of the desiredquality. Computer 44 provides signal 15, in accordance with thefollowing equation:

B n TEM where m and n are constants hereinafter described in detail andT is the extract-mix temperature in tower 3. For furfural solvent, m andn have values of 1.33 and 0.5, respectively.

Signal E is applied to a logarithmic amplifier 45 which is part of anexponential circuit 46 that also includes a multiplier 47, anoperational amplifier 50 and a feedback element 51. The output oflogarithmic amplifier 45 is applied to multiplier 47 where it ismultiplied with a direct current voltage V; from source 36 correspondingto the term (m-n) in equation 5. Feedback element 51, which may be afunction generator of the type manufactured by Electronics Associates astheir part PC-l2, cooperates with operational amplifier 50 to providethe antilog of the output of multiplier 47 so that the output ofexponential circuit 46 corresponds to S""'". A multiplier 54 multipliesa direct current voltage V, from source 36, which corresponds to theconstant A of the charge oil, with the output from exponential circuit46. Subtracting means 55 subtracts temperature signal E, from a directcurrent voltage V from source 36, corresponding to the term T inequation 5. A divider 56 divides the output from multiplier 54 by theoutput from subtracting means 55 to provide a signal to a sample andhold circuit 58. When the refined oil is of the proper quality, a switch60, which may be of the mometary on/off pushbutton type, is activated.When activated, switch 60 passes a direct current voltage V, from source36 to sample and hold circuit 58 causing circuit 58 to sample and holdthe output from divider 56. Circuit 58 provides signal E which remainsconstant for the remainder of the operation even though inputs tocomputer 44 may vary.

Referring to FIGS. 1 and 5, a signal E corresponding to the actualsolvent selectivity factor C is provided by a C computer 62 inaccordance with signals E E from computers 32 and 44, respectively, adirect current voltage V from source 36, and the following equation:

u Sm (6) An exponential circuit 46A receives signal E and voltage Vwhich correspond to the term m in equation 6, and provides a signalcorresponding to the term S" to a divider 63. Divider 63 divides signalE from computer 44 with the output from exponential circuit 46A toprovide signal E Referring to FIG. 1 summing means 66 and a divider 67operate as a computer to provide a signal E in accordance with equation1, which corresponds to the limiting value of the selectivity factor Cdue to the maximum extract oil flow rate line shown in FIG. 2. Summingmeans 66 sums direct current voltages V,, V from source 36. Directcurrent voltage V corresponds to the maximum extract oil flow rate asdetermined from the physical design of the refining unit. Divider 67divides voltage V with the output from summing means 66 to providesignal E Referring to FIGS. 1 and 6, a C computer 70 provides a signal Ecorresponding to the value of the limitation on the solvent selectivityfactor C for the maximum refined oil flow rate R0,, in accordance with Edirect current voltages V V and V and equation 2. Multipliers 72, 73multiply direct current voltages V V with voltage V and signal Erespectively. Voltage V corresponds to the maximum possible refined oilflow rate. Subtracting means 75 subtracts the output from multiplier 73from the output from multiplier 72 to provide signal corresponding tol00(SOLS(ROM). Multiplier 76 multiplies signal Em with voltage V, toprovide a signal to summing means 79, where the signal is summed withthe output from subtracting means 75 to provide a signal correspondingto the denominator in equation 2. A divider 80 divides the output ofsubtracting means 75 with the output from summing means 79 to providesignal E Referring to FIGS. 1 and 7, a computer provides a signal Ecorresponding to the value of the limitation on the solvent selectivityfactor C for the maximum operating temperature for the particular dosagerate. Computer 85 provides signal E in accordance with signal E fromcomputer 32, direct current voltage V V and V and equation 3. A squareroot circuit 86 receives signal E from S computer 32 and provides asignal corresponding to S". Subtracting means 87 subtracts voltage Vwhich correspond to the misciblity temperature, from voltage V toprovide a signal corresponding to (T T The output from subtracting means87 is multiplied with the output from square root circuit 86 by amultiplier 90. A divider 91 divides direct current voltage V.,,corresponding to the A constant, with the output from multiplier toprovide signal E Signals E E and E from divider 67 and computers 70 and85, respectively, are applied to comparators 94, 94A and 94B,respectively, where they are compared with signal E from C computer 62.Comparators 94, 94A and 94B perform the function of ascertaining thatthe actual solvent selectivity characteristic C lies within the feasibleoperating area shown in Graph 2. When signal E is equal to or less thansignals E E and E each comparator provides a high level direct currentoutput. When signal E is greater than a signal, either E E or E thecorresponding comparator provides a low level direct current output. Theoutputs from comparators 94 through 94B are applied to a NOR gate 125,which controls the changing of the set point in flow recorder controller6, as hereinafter explained.

Referring to FIGS. 1 and 8, a computer 98 provides a temperature Tsignal E to switch 30 which is used to adjust the set point oftemperature recorder controller 14 as hereinafter explained. Signal E isprovided in accordance with signals E E from computer 32 and 44,respectively, direct current voltage V V and V from source 36 and thefollowing equation:

An exponential circuit 64B receives signal E from computer 32 andvoltage V; from source 36 and provides a signal corresponding to S"""".A multiplier 100 multiplies the output from exponential circuit 46B withdirect current voltage V, to provide a signal corresponding to the termAS" in equation 7. The signal from multiplier 100 is divided by signal Efrom computer 44 by a divider 102 to provide a signal, corresponding toAS""""/B, to subtracting means 103. Subtracting means 103 subtracts thesignal from divider 102 from voltage V corresponding to the referencetemperature T for the solvent, to provide signal E In determining theoptimum yield, it is necessary to provide signals corresponding to anearnings position for the refining unit. Referring to FIGS. 1 and 9,earnings computer 110 provides a signal E corresponding to the currentearnings position EP in accordance with signals E E from flowtransmitters 20 and 20A, respectively, direct current voltages V V and Vfrom source 36 and the following equation:

EP (Value of refined oil)(RO)+Value of extract oil(E)-(Value of chargeoil)(RO-i-EO). s

Multipliers 111, 112 multiply signals E and E respectively, withvoltages V and V respectively, which correspond to the value of therefined oil and the extract oil, respectively. Summing means 114 sumsthe outputs from multiplier 111, 112, while summing means 115 sumssignals E E to provide a signal corresponding to the charge oil flowrate to a multiplier 116. The output from summing means 1 is multipliedwith voltage V which corresponds to the value of the charge oil.Subtracting means 120 subtracts the output from multiplier 1 16 from theoutput of summing means 114 to provide signal E Signal E is applied to asample and hold circuit 122 and to a comparator 123. Sample and holdcircuit 122 provides a signal corresponding to the earnings position forthe next previous set of conditions, as hereinafter explained.Comparator 123 compares the current earnings position with the previousearnings position to determine whether the earning position hasincreased or decreased. When signal E from computer 110 is equal to orgreater than the output from sample and hold circuit 122, comparator 123provides a high level direct current output to NOR gate 125 which alsoreceives the outputs from comparators 94 through 94B. When signal E fromcomputer 1 10 is less than the output from sample and hold circuit 122,comparator 123 provides a low level direct current output. When acomparator 94, 94A, 94B or 123 provide a low level direct currentoutput, NOR gate 125 provides a high level direct current output. Whencomparators 94, 94A, 94B and 123 provide high level direct currentoutputs, NOR gate 125 provides a low level direct current output.

The output from NOR gate 125 is applied to switch 30 which passes theoutput to flow recorder controller 6 when activated and blocks theoutput when not activated. When a low level direct current output fromNOR gate 125 is applied to controller 6, the set point may be changed soas to increase the charge oil flow rate in line 4. When a high leveldirect current output from NOR gate 125 is applied to controller 6, theset point may be changed so as to decrease the charge oil flow rate inline 4.

The output from NOR gate 125 is also applied to AND gate 127 which ispart of a circuit for changing the set point of flow recorder controller6. The circuit further includes monostable multivibrators and 135A, aflip-flop 136, clock means 138 and 139, AND gates and 140A, and a switch144, which may a momentary on pushbutton switch. Switch 144 is activatedat the beginning of the refining operation to momentarily pass a directcurrent voltage V from source 36 to flip-flop 136. The direct currentvoltage triggers flip-flop 136 to a clear state causing its Q output tobe a high level direct current voltage. The high level Q output fromflip-flop 136 enables AND gate 140.

When disabled, AND gate 140 effectively blocks pulses from clock means138. When enabled, AND gate 140 effectively passes the pulses from clockmeans 138. The pulse repetition rate of the pulses from clock means 138is such that the refining unit reaches a steady state condition after achange in response to a pulse from clock means 138 before clock means138 provides another pulse.

Each pulse passed by AND gate 140A controls circuit 122 to sample andhold the current earnings position signal E so as to provide a signalcorresponding to the next previous earning position for the next stepincrease in the charge oil flow rate.

Each pulse passed by AND gate 140 triggers monostable multivibrator 135Ato provide an enabling pulse to AND gate 140A. When disabled, AND gate140A effectively blocks pulses provided by clock means 139. Whenenabled, AND gate 140A effectively passes the pulses from clock means139. Each pulse passed by AND gate 140, when applied to flow recordercontroller 6, changes the set point of controller 6 a predeterminedamount in a direction controlled by the output from NOR gate 125. Thewidth of the pulse from multivibrator 135A controls the number of pulsesfrom clock means 139 that are effectively passed by AND gate 140A so asto control the amount of change of the charge oil flow rate. The passedpulses from AND gate 140A are applied to switch 130.

When the refining unit has reached a stabilized condition after beinginitially operated at the selected solvent dosage and temperaturecombination, switch 30 is activated so that the refining unit iscontrolled by the system of the present invention. The charge oil flowrate is periodically increased in a stepping fashion, due to the passedpulses from AND gate 140A and the output from NOR gate 125, so as todecrease the solvent dosage. Switch 30 also passes signal E from T Mcomputer 98 to temperature recorder controller 14 so as to control theset point of temperature recorder controller 14. Temperature recordercontroller 14 changes the temperature of the extract-mix leaving tower 3so that the proper temperature is provided for each new solvent dosage.

The charge oil flow rate in line 4 increases in a stepping fashion untilthe actual solvent selectivity characteristic exceeds one of itslimitations, or the current earnings position for the refining unit isless than the next previous earnings position. Thus, when signal E isgreater than signal E E or E or signal E is less than the output fromsample and hold circuit 122, a comparator 94, 94A, 948 or 123 provides alow level direct current output to NOR gate 125 causing NOR gate 125 toprovide a high level direct current output. The change in level ofoutput from NOR gate 125 triggers multivibrator 135 which acts as a timedelay. Multivibratorl35 provides a pulse whose trailing edge triggersflip-flop 136 to a set state. The Q output from flipflop 136 goes to alow level thereby disabling AND gate 140. Due to the time delay effectof the width of the pulse from multivibrator 135, one more pulse fromclock means 138 is effectively passed by AND gate 140. That pulse causesa change in the set point of flow recorder controller 6, as heretoforeexplained. However, the change in the set point is in the oppositedirection, since a high level direct current output from NOR gate 125 isbeing applied to flow recorder controller 6. The change in the set pointof flow recorder controller 6 causes a step decrease in the charge oilflow rate, thereby increasing the solvent dosage, so that the refiningunit has been returned to the next previous operating step.

Flip-flop 136 remains in the set state, until switch 144 is activated,so that the refining unit is maintained at an operating condition foroptimum yields of refined oil and extract oil.

Although the system of the present invention has been shown using analogcomputers, a digital computer may be used. Analog-to-digital converterswould convert signals E E E to digitals signals and apply them to thedigital computer. Information relating to the maximum possible flowrates of the solvent, the refined waxy oil and the extract oil may beprogrammed into the digital computer. Digital-to-analog converters wouldconvert digitals signals from the digital computer to analog signals soas to provide signals E through E-, and E to control the refining unit.The digital computer would use the aforementioned equations in providingsignals B, through E, and E The system of the present invention, asheretofore described, controls a solvent refining unit to provideoptimum yields of refined oil and extract oil from charge oil using therelationship of a solvent selectivity characteristic and the solventdosage along with the earnings position of the solvent refining unit.The system of the present invention determines a quality constant forthe charge oil being refined in a solvent refining unit and uses thequality constant to control the refining operation.

DERIVATIONS OF EQUATIONS 1 THROUGH 7 The following equation wasdisclosed in US Pat. No. 3,458,432 issued July 29, 1969 to R. A. Woodleet al. and assigned to Texaco Inc., assignee of the present invention.

e m (9) where S is the term F in the aforementioned patent, C, is thesolvent selectivity characteristic which defined as the amount ofextract oil in the extract mix and may be written in equation form as nEM1) el q n"' EM2) e2 q where q is a constant for a given solventdosage, and C C are solvent selectivity characteristics for extract-mixtemperatures T and T respectively. Equations 1 1, 12 may be rewritten as(ll)and R el am C21: R e2 am ez n er R e2 EMr er EM2 e2 solving for Tequation 14 is written as T Trum n EM2 e2 R el e?! Using FIG. 10, whichis the extract mix temperature versus solvent selectivity characteristiccurve for furfural solvent and a particular charge oil, temperatures T Tare 169F and 202F, respectively, and the selectivity characteristics C Care 0.0854 and 0.1087, respectively. From equation 15, the resultingreference temperature is 322.9F, which is rounded off to 323F. Thedashed line is a reference line; the temperature value of the dashedline is the reference temperature T for the furfural solvent curve.Similarly curves may be drawn for other solvent types from experimentaldata and their reference temperatures may be determined accordingly.

As heretofore mentioned, the sumof the extract oil and refined oil flowrates is equal to the charge oil flow rate. It follows that E0 C0 R0(16) where CO and R0 are the charge oil and refined oil flow rates.Substituting for E0 in equation 10, equation 10 may be rewritten as isdone in the following equation:

Solving equation 4 for CO and substituting in equation 17, equation l2may be rewritten as S =S(SOL) +100(SOL) S(RO) B C S 20 The exponent kmay be written as l/m and equation 20 can then be written as(B)"'=B=C,,S"' 21 Equation 5 is obtained by substituting for C inequation 21 from equation 9. Equation 6 is a rewriting of equation whereC, is designated as C for convenience. Equation 7 is a rewriting ofequation 5, solving for the temperature.

What is claimed is:

l. A control system for a solvent refining unit which treats charge oilwith a solvent in a refining tower to yield raffinate and extract-mix,strippers separate the solvent from the raffinate and from theextract-mix to provide refined waxy oil and extract oil, respectively,the solvent is returned to the tower and the refined waxy oil issubsequently dewaxed to provide refined oil, comprising means forproviding a signal corresponding to a quality constant for a desiredquality refined oil, means for sensing some of the refining conditionsand providing signals corresponding thereto, means connected to thesensing means for providing signals corresponding to limitations of arefining parameter in accordance with at least one sensed conditionsignal, means connected to the sensing means and to the quality constantsignal means for providing a signal corresponding to the actual value ofthe refining parameter in accordance with the quality constant signaland some of the sensed condition signals, means connected to the sensingmeans for providing an earnings signal corresponding to the relationshipof a present earnings position of the refining unit with a previousearnings position in accordance with some of the sensed conditionsignals and current values of the charge oil, the refined oil and theextract oil, and means connected to the limitation signal means, to thequality constant signal means, to the refining parameter signal meansand to the earnings signal means for controlling the refining unit toprovide optimum yields of the desired quality refined oil and extractoil from the charge oil in accordance with the quality constant signal,the refining parameter signal, the earnings signal and the limitationsignals.

2. A system as described in claim 1 in which the sensed refiningconditions are the flow rates E0, R0 of the extract oil and refined waxyoil, respectively, and the temperature T of the extract mix in therefining tower; the control means controls the refining unit bycontrolling the flow rate of the charge oil and the extract-mixtemperature T and the refining parameter is a solvent selectivitycharacteristic C.

3. A system as described in claim 2 in which the quality constant signalmeans is connected to the sensing means and provides the qualityconstant B signal in accordance with the sensed extract-mix temperatureT signal and the following equation:

where A is a characteristic of the charge oil S is the solvent dosage inpercent volume, m and n are characteristic constants related to the typeof solvent, and T is a reference temperature for the solvent.

4. A system as described in claim 3 in which the limitation signal meansprovides signals, corresponding to limitations C C and C on the solventselectivity characteristic C, as the limitation signals in accordancewith the following equations:

where EO RO and SOL are the maximum possible flow rates, for therefining unit, of the extract oil, the refined oil and the solvent,respectively, and T is the miscibility temperature for the solvent.

5. A system as described in claim 4 in which solvent selectivitycharacteristic signal means provides the solvent selectivity C signal inaccordance with the following equation:

7. A method of controlling a solvent refining unit which treats chargeoil with a solvent in a refining tower to yield raffinate andextract-mix, strippers separate the solvent from the raffinate and fromthe extract-mix to provide refined waxy oil and extract oil,respectively, the solvent is returned to the tower and the refined waxyoil is subsequently dewaxed to provide refined oil, which comprisesdetermining the quality constant for a desired quality refined oil,sensing some of the refining conditions, determining the limitations ofa refining parameter in accordance with at least one sensed refiningcondition, determining the actual value of the refining parameter inaccordance with the quality constant and some of the sensed refiningconditions, determining the relationship of a present earnings posi tionof the refining unit with a previous earnings position in accordancewith some of the sensed refining conditions and current values of thecharge oil, the refined oil and the extract oil, and controlling therefining unit to provide optimum yields of the desired quality refinedoil and extract oil from the charge oil in accordance with the qualityconstant, the earnings relationship, the actual value of the refiningparameter, and the limitations on the refining parameter.

8. A method as described in claim 7 in which the sensed refiningconditions are the flow rates EO, R of the extract oil and the refinedwaxy oil, respectively, and the temperature T of the extract mix; therefining unit is controlled by controlling the flow rate of the chargeoil and the temperature T and the refining parameter is a solventselectivity characteristic C.

9. A method as described in claim 8 in which the quality constant isdetermined in accordance with the sensed extract-mix temperature T andthe following equation:

AShn-n) TFTR where A is a characteristic of the charge oil, S is thesolvent dosage, m and n are characteristic constants related to the typeof solvent, and T is a reference temperature for the solvent.

10. A method as described in claim 9 in which there are threelimitations C C and C on the solvent selectivity characteristic andwhich are determined in accordance with the following equations:

A CT(TR" TMIBC)SD 12. A method as described in claim 11 in which thecontrolling step includes increasing the flow rate of the charge oil insteps until the actual value of the solvent selectivity characteristicexceeds at least one of the limitations on the solvent selectivitycharacteristic, or the earnings relationship indicates that the presentearnings position of the refining unit is less than the earningsposition for the next previous step, decreasing the charge oil flow rateso as to avoid the limitations and the lesser earnings position, andcontrolling the temperature T of the extract-mix in accordance with thefollowing equation:

1. A control system for a solvent refining unit which treats charge oilwith a solvent in a refining tower to yield raffinate and extract-mix,strippers separate the solvent from the raffinate and from theextract-mix to provide refined waxy oil and extract oil, respectively,the solvent is returned to the tower and the refined waxy oil issubsequently dewaxed to provide refined oil, comprising means forproviding a signal corresponding to a quality constant for a desiredquality refined oil, means for sensing some of the refining conditionsand providing signals corresponding thereto, means connected to thesensing means for providing signals corresponding to limitations of arefining parameter in accordance with at least one sensed conditionsignal, means connected to the sensing means and to the quality constantsignal means for providing a signal corresponding to the actual value ofthe refining parameter in accordance with the quality constant signaland some of the sensed condition signals, means connected to the sensingmeans for providing an earnings signal corresponding to the relationshipof a present earnings position of the refining unit with a previousearnings position in accordance with some of the sensed conditionsignals and current values of the charge oil, the refined oil and theextract oil, and means connected to the limitation signal means, to thequality constant signal means, to the refining parameter signal meansand to the earnings signal means for controlling the refining unit toprovide optimum yields of the desired quality refined oil and extractoil from the charge oil in accordance with the quality constant signal,the refining parameter signal, the earnings signal and the limitationsignals.
 2. A system as described in claim 1 in which the sensedrefining conditions are the flow rates EO, RO of the extract oil andrefined waxy oil, respectively, and the temperature TE.M. of the extractmix in the refining tower; the control means controls the refining unitby controlling the flow rate of the charge oil and the extract-mixtemperature TE.M.; and the refining parameter is a solvent selectivitycharacteristic C.
 3. A system as described in claim 2 in which thequality constant signal means is connected to the sensing means andprovides the quality constant B signal in accordance with the sensedextract-mix temperature TE.M. signal and the following equation: where Ais a characteristic of the charge oil S is the solvent dosage in percentvolume, m and n are characteristic constants related to the type ofsolvent, and TR is a reference temperature for the solvent.
 4. A systemas described in claim 3 in which the limitation signal means providessignals, corresponding to limitations CEO, CRO and CT on the solventselectivity characteristic C, as the limitation signals in accordancewith the following equations:
 5. A system as described in claim 4 inwhich solvent selectivity characteristic signal means provides thesolvent selectivity C signal in accordance with the following equation:C B/Sm
 6. A system as described in claim 5 in which the control meansincreases the flow rate of the charge oil in steps until the solventselectivity characteristic signal is greater than at least one of thelimitation signals, or the earnings signal indicates that the presentearnings position of the refining unit is less than the earningsposition for the next previous step, at which time the control meansdecreases the charge oil flow rate and controls the temperature T of theextract-mix in accordance with the following equation:
 7. A method ofcontrolling a solvent refining unit which treats charge oil with asolvent in a refining tower to yield raffinate and extract-mix,strippers separate the solvent from the raffinate and from theextract-mix to provide refined waxy oil and extract oil, respectively,the solvent is returned to the tower and the refined waxy oil issubsequently dewaxed to provide refined oil, which comprises determiningthe quality constant for a desired quality refined oil, sensing some ofthe refining conditions, determining the limitations of a refiningparameter in accordance with at least one sensed refining condition,determining the actual value of the refining parameter in accordancewith the quality constant and some of the sensed refining conditions,determining the relationship of a present earnings position of therefining unit with a previous earnings position in accordance with someof the sensed refining conditions and current values of the charge oil,the refined oil and the extract oil, and controlling the refining unitto provide optimum yields of the desired quality refined oil and extractoil from the charge oil in accordance with the quality constant, theearnings relationship, the actual value of the refining parameter, andthe limitations on the refining parameter.
 8. A method as described inclaim 7 in which the sensed refining conditions are the flow rates EO,RO of the extract oil and the refined waxy oil, respectively, and thetemperature TEM of the extract mix; the refining unit is controlled bycontrolling the flow rate of the charge oil and the temperature TEM; andthe refining parameter is a solvent selectivity characteristic C.
 9. Amethod as described in claim 8 in which the quality constant isdetermined in accordance with the sensed extract-mix temperature TEM andthe following equation: where A is a characteristic of the charge oil, Sis the solvent dosage, m and n are characteristic constants related tothe type of solvent, and TR is a reference temperature for the solvent.10. A method as described in claim 9 in which there are threelimitations CEO, CRO and CT on the solvent selectivity characteristicand which are determined in accordance with the following equations: 11.A method as described in claim 10 in which the actual value of thesolvent selectivity characteristic is determined in accordance with theequation: C B/Sm
 12. A method as described in claim 11 in which thecontrolling step includes increasing the flow rate of the charge oil insteps until the actual value of the solvent selectivity characteristicexceeds at least one of the limitations on the solvent selectivitycharacteristic, or the earnings relatioNship indicates that the presentearnings position of the refining unit is less than the earningsposition for the next previous step, decreasing the charge oil flow rateso as to avoid the limitations and the lesser earnings position, andcontrolling the temperature TEM of the extract-mix in accordance withthe following equation: